Heretofore, vehicle theft prevention systems, such as a keyless entry system, a smart entry system, a smart start system, etc. have been employed for the purpose of preventing vehicles from being stolen.
The keyless entry system and the smart entry system refer to a system for locking and unlocking vehicle doors based on wireless communications between a portable device and a vehicle-mounted authenticating device without the need for inserting a key into a cylinder that is mounted in the door.
According to the keyless entry system, when the user presses a button on the portable device, the portable device sends an authentication code to the vehicle-mounted authenticating device, which checks the authentication code against a reference code stored in the vehicle-mounted authenticating device. If the check result satisfies a given condition, e.g., if the authentication code and the reference code are in full agreement with each other, then the doors are locked or unlocked.
According to the smart entry system, when a trigger condition is satisfied, e.g., when the user touches a door handle of the vehicle, the vehicle-mounted authenticating device sends a signal for requesting an authentication code to the portable device. In response to the request signal, the portable device sends the authentication code to the vehicle-mounted authenticating device, which checks the authentication code against a reference code stored in the vehicle-mounted authenticating device. If the check result satisfies a given condition, e.g., if the authentication code and the reference code are in full agreement with each other, then the doors are locked or unlocked. The smart entry system makes it possible to lock and unlock the doors though the user does not operate a button on the portable device.
The smart start system refers to a system for permitting an engine to be started based on wireless communications between a portable device and a vehicle-mounted authenticating device without the need for inserting a key into a cylinder. According to the smart start system, when the driver turns an ignition knob, the vehicle-mounted authenticating device sends a signal for requesting an authentication code to the portable device. In response to the request signal, the portable device sends the authentication code to the vehicle-mounted authenticating device, which checks the authentication code against a reference code stored in the vehicle-mounted authenticating device. If the check result satisfies a given condition, e.g., if the authentication code and the reference code are in full agreement with each other, then the engine is permitted to start.
In each of the above systems, as described above, wireless communications take place between the portable device and the vehicle-mounted authenticating device. If the signal sent from the portable device to the vehicle-mounted authenticating device is of a fixed content, then when the signal from the portable device is intercepted by a third party and the third party separately sends an identical signal to the vehicle-mounted authenticating device, the vehicle-mounted authenticating device misidentifies the signal as being sent from the portable device, and permits the vehicle doors to be unlocked. As a result, the vehicle may possibly be stolen.
One of the technologies developed to reduce the above possibility is a rolling code (see Japanese Laid-Open Patent Publication No. 01-278671, Japanese Laid-Open Patent Publication No. 08-102982, and Japanese Laid-Open Patent Publication No. 10-061277). The rolling code is a code that is regularly renewed each time wireless communications take place from the portable device to the vehicle-mounted authenticating device. For example, a rolling code that is renewed according to a function formula: f(n+1)=f(n)+1 is incremented by 1 for each wireless communications session, and hence is changed at all times. The function formula and the preceding rolling code are stored in the portable device and the vehicle-mounted authenticating device. Even if a signal from the portable device is intercepted and an identical signal is sent to the vehicle-mounted authenticating device, the vehicle-mounted authenticating device does not permit the doors to be unlocked because the received signal is different from the value of the appropriate rolling code.
With the keyless entry system, for example, the button on the portable device may be pressed when the portable device is so away from the vehicle-mounted authenticating device that no wireless communications are possible therebetween, or an unexpected communication error may happen. Thus, the vehicle-mounted authenticating device not only authenticates a received signal which is the same as the value of the appropriate rolling code, but also authenticates a received signal with a certain degree of latitude. For example, if the value of the appropriate rolling code is 100, then a signal having a value in the range from 100 to 120 is judged as a rolling code sent from an authentic portable device, and the doors are permitted to be unlocked.
Various attempts have been made to increase the security of the rolling code.
According to Japanese Laid-Open Patent Publication No. 08-102982, a fixed ID code and a variable rolling code are not sent as separate bits, but an ID code is varied depending on a rolling code. Since the ID code itself is variable, fixed portions are reduced for increased security.
According to Japanese Laid-Open Patent Publication No. 10-061277, a key code inherent in the vehicle which the designer is not informed of is used to encrypt a rolling code for increased security.